1. Field of the Invention
The present invention relates to a light control film which controls emission angle from a light source, and a lighting device and a display device using the same. In particular, the present invention relates to a light control film which emits light from the light source with high efficiency, and a lighting device and a display device using the same.
2. Description of Related Art
In recent years, an optical film (light control film) which is provided on a viewer side of a display or in between a liquid crystal display and a light source so as to control the emission angle is widely used in order to prevent a mobile telephone or a mobile PC from being peeped or to prevent reflection on the front glass of a car. As a kind of a light control film, Japanese Patent Application Laid-Open No. 2001-242797 (Patent Document 1) (pp. 2-5, FIG. 1) proposes a film in which portions of different refractive indexes are arranged alternately.
Patent Document 1 does not describe incident light on a film, emitted light from a film and light paths in a film, so details are unknown. However, in such a film as shown in Patent Document 1, if the light incident angle distribution on a film is large, light near the ultimate angle is made incident on the emitting surface, whereby light having wide emission angle is obtained. Further, light having larger angle than the ultimate angle returns to a light source direction, whereby the emission efficiency is easily lowered. Accordingly, if the incident distribution is large in particular, it is difficult for the film of Patent Document 1 to control emitted light and to realize high emission efficiency.
Further, Japanese Patent Publication No. 06-027974 (Patent Document 2) and Japanese Patent Publication No. 06-005462 (Patent Document 3) propose backlights in each of which a film having the same configuration as that of Patent Document 1 is disposed between a liquid crystal display and a light source. As described in Patent Document 2 (pp. 1-3, FIG. 3), such a backlight can cause light emitted from the light source at a certain angle to reflect on a film so as to be emitted in an emitting direction as a backlight. However, if the incident light angle distribution on a film is large, light transmitting the refractive index interface is caused, whereby it is difficult to control emitted light. Further, in general, in a cold cathode tube or an LED (light emission diode) used as a light source, pieces of light having various emission angles are emitted from one point of the light source, so the proportion of pieces of light having a certain angle as described in FIG. 3 of Patent Document 2 is small. Therefore, with the configuration shown in Patent Documents 2 and Patent Document 3 (pp. 1-3, FIG. 1) the controllable amount of light becomes small, so it is difficult to control light with high efficiency.
As a light control film, there is one having a structure in which transparent layers and shielding layers are arranged alternately, besides a structure in which layers of different refractive indexes are arranged alternately as described in Patent Documents 1 to 3.
For example, as described in Japanese Patent No. 3043069 (Patent Document 4) (p. 1, pp. 3-5, FIGS. 1 and 2), there is a film in which a center area includes a shielding layer having relatively high light absorption rate and outside areas contacting the transparent area and having relatively low light absorption rate, in order to prevent ghost image.
However, in a light control film using such shielding layers, the emitting direction and the emission angle of emitted light are controlled by controlling the thickness of the film and the width of a transparent layer sandwiched between shielding layers. This enables to control the emission angle strictly, but causes a problem of loss of light for the amount of the light absorbed in the shielding layers.
As a means to solve this problem, a film described in Japanese Patent Application Laid-Open No. 2004-062084 (Patent Document 5) (pp. 2-5, FIG. 1) has been proposed. The film described in Patent Document 5 is so configured that a layer having smaller refractive index than that of a louver layer (a layer in which transparent layers and shielding layers (light absorption layers) are arranged alternately) is provided on the light incident side to thereby suppress loss of light.
Further, Japanese Patent Application Laid-Open N 2004-020725 (Patent Document 6) (pp. 2-7, FIG. 2) proposes a film in which a light shielding layer is tilted and one surface of the light shielding layer consists of a light reflective-type shielding layer and the other surface consists of a light absorbing type shielding layer. In such a film, light reflected off the light reflective-type shielding layer returns to the light source side, and the light is reused so as to improve the light use efficiency and to prevent the luminance of the monitor from being lowered.
Further, Japanese Patent Application Laid-Open No. 2004-245918 (Patent Document 7) (pp. 2-8, FIG. 3) also proposes a film using absorbing layers and reflective layers. In Patent Document 7, by using a reflective layer contacting one surface of the transparent layer and an absorbing layer contacting the other surface, emission angle distribution from the film is made asymmetry so as to prevent reflection to thereby improve visibility in a state where the sheet is reclined.
Japanese Patent Application Laid-Open No. 2004-109615 (Patent Document 8) (pp. 2-7, FIGS. 1 and 2) describes a film in which all shielding layers consist of reflective layers. Further, Japanese Utility-Model Laid-Open No. 06-076934 (pp. 2-4, FIG. 1) proposes a liquid crystal display capable of preventing reflection and realizing transmission display and reflection display, by providing a reflective film, which transmits backlight and reflects outside light, on the backlight incident surface side of the light control film.
Further, Japanese Patent Application Laid-Open No. 06-094902 (Patent Document 10) (pp. 2-6, pp. 14-15, FIGS. 1 and 7) proposes an optical element having such a configuration that unit elements in each of which the surrounding of a refractive optical material is coated with a low refractive index material are arranged two-dimensionally via a light absorbing material. Further, Japanese Patent Application Laid-Open No. 2003-066206 (Patent Document 11) (pp. 2-4, FIG. 3) proposes a view enlarging member in which the emitting tilt surface of a trapezoidal lens is covered with a low refractive index layer and a part between tilt surfaces is filled with a light absorbing layer.
However, the structure in which layers of different refractive indexes are arranged alternately, described in Patent Documents 1 to 3, has the following problem. That is, if light incident distribution on a film is large, it is difficult to realize a sufficient control of emission angle and high emission efficiency, and the controllable amount of light is limited.
Further, the film described in Patent Document 4 involves a problem of causing a loss of light for the amount of light absorbed in the shielding layers. The film described in Patent Document 5, proposed to solve this problem, requires a low refractive index layer besides a louver layer, so the film is likely to be thick, and the effect thereof is 1.2 times that of a film without a low refractive index layer, which is small.
Further, in the film described in FIG. 2 of Patent Document 6, light reflected off a light reflective-type shielding layer may be absorbed in the light absorbing-type shielding layer, so light to be reused is reduced. Thereby, the effect of improving the light use efficiency becomes small. In the film described in Patent Document 7, the emission angle distribution from the film becomes asymmetry, so the film cannot be used for a device requiring symmetrical emission angle distribution like mobile equipment. Further, in asymmetrical emission angle distribution, light of the large emitted light distribution side (in Patent Document 7, a direction viewable in a state where the sheet is reclined) is not controlled, which causes light emitted to a waste angle.
Further, in the optical element described in FIG. 1 of Patent Document 10, light from the light source is also made incident on a light absorbing member, so the light use efficiency of the optical element largely drops. In order to solve this problem, Patent Document 10 proposes an element having a lens structure on the light source side of the optical element as shown in FIG. 7 of Patent Document 10. With this structure, the element becomes thick as a whole, so it is difficult to use it in mobile equipment which is required to be thinner.
Further, the element described in FIG. 3 of Patent Document 11 is intended to enlarge light from the light source (liquid crystal panel), different from Patent Documents 1 to 10, so the emission angle cannot be controlled (emitted light distribution from the light source cannot be narrowed). Further, the light absorbing layer in Patent Document 11 is a layer for absorbing stray light and improving the contrast, as described in paragraph [0011] of the specification thereof, so it has no relation with a control of emission angle.